You need to run apt-get update first to download the current state of the package repositories. Docker images do not include this to save space, and because they’d likely be outdated when you use it. If you are doing this in a Dockerfile, make sure to keep it as a single RUN command so that caching of the layers doesn’t cache an old version of the update command with a new package install request:
RUN apt-get update \
&& DEBIAN_FRONTEND=noninteractive apt-get install -y \
net-tools \
&& apt-get clean \
&& rm -rf /var/lib/apt/lists/*